"We screened subjects for factors such as income and education, but there may be other factors such as lifestyle and nutritional intake."

In particular, he pointed out that those who drank sake tended to eat larger quantities of raw fish, which is thought to have possible benefits to cognitive function, while wine drinkers tended to eat more cheese, which may have a similar effect.

Dr Jonathan Chick, a senior lecturer in psychiatry at Edinburgh University, echoed this, saying that other studies had noted this association between moderate alcohol and brain function, but could not prove that the alcohol was definitely the reason.

"It's so difficult to take other factors into account - for example, moderate drinkers might have a better social environment," he told BBC News Online.

"I don't think I would want to tell somebody who isn't drinking to start, as they may be teetotal for very good reasons.

"However, for someone who has had a stroke due to blood clotting, I would not tell them to stop drinking moderately."

However, he spelled out the dangers of heavy drinking, particularly binge drinking, which is know to actually reduce brain power.